1. Field of the Invention
The invention relates generally to optical spectrometers and to the use of a Littrow configuration spectrometer in a spectral domain optical coherence tomography system.
2. Description of Related Art
Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time. In recent years, it has been demonstrated that spectral domain OCT has significant advantages in speed as compared to time domain OCT. In spectral domain OCT, the optical path length difference between the sample and reference arm is not mechanically scanned but rather the interferometrically combined beam is sent to a spectrometer in which different wavelength components are dispersed onto different photodetectors to form a spatially oscillating interference fringe (Smith, L. M. and C. C. Dobson (1989). “Absolute displacement measurements using modulation of the spectrum of white light in a Michelson interferometer.” Applied Optics 28(15): 3339-3342). A Fourier transform of the spatially oscillating intensity distribution can provide the information of the reflectance distribution along the depth within the sample. As there is no mechanical depth scanning, acquisition of light reflection along a full depth range within the sample can be achieved simultaneously, and consequently, the speed of obtaining a full depth reflection image is substantially increased as compared to time domain OCT (Wojtkowski, M., et al. (2003). “Real-time in vivo imaging by high-speed spectral optical coherence tomography.” Optics Letters 28(19): 1745-1747; Leitgeb, R. A., et al. (2003). “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography.” Optics Letters 28(22): 2201-2203). In addition, as the light reflected from the full depth range within the sample is fully dispersed over many photodetectors, the shot noise for each photodetector is substantially reduced as compared to the time domain OCT case, and hence the signal to noise ratio can also be substantially increased (Leitgeb, R. A., et al. (2003). “Performance of Fourier domain vs. time domain optical coherence tomography.” Optics Express 11(8): 889-894; De-Boer, J. F., et al. (2003). “Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography.” Optics Letters 28(21): 2067-2069; Choma, M. A., M. V. Sarunic, et al. (2003). “Sensitivity advantage of swept source and Fourier domain optical coherence tomography.” Optics Express 11(18): 2183-2189)
In most high speed spectral domain optical coherence tomography (SD-OCT) designs, a spectrometer is used with a diffraction grating that disperses the incident beam into its spectral components and a detector array that receives the dispersed spectral components. Typically, an SD-OCT system uses the classical or in-plane diffraction configuration with the incident and diffracted beams all perpendicular to the grating grooves (see for example: JP2000-046729, U.S. Pat. No. 5,565,986, JP2000-046729, JP2001-174404, WO03062802 (US20050018201), U.S. Pat. No. 2,476,174, WO2004043245, US20040239938 (WO2004111929)). In such a case, the dispersed spectral component beams are co-planar with the incident beam and can thus be relatively easily focused into a line to shine onto a detector array for detection of the interference spectrum. However, due to the large extent of the dispersed spectrum in the plane of the incident and dispersed beams, the incident and diffracted beams generally do not share a common lens, as doing so would put the incident beam at a large off-axis angle, which tends to cause distortion in the incident beam and this distortion carries through to the diffracted beams. In order to reduce the off-axis distortion of both the diffracted beams and the incident beam, two separate lenses are generally used and a relatively large angle between incidence and diffraction is required. A problem associated with such a design is that the size of the spectrometer will be large and in addition, when a standard high speed (>1000 lines/sec) line scan camera is used as the detector array, owing to the small height of the linear array pixels (about 10 microns), the spectrometer output can be very sensitive to the tip movement of the focused spectral line with respect to the linear array pixels caused by mechanical vibration and temperature variation.
In order to reduce the size of the spectrometer, a Littrow configuration can be used. The term Littrow configuration is sometimes used to describe an arrangement where the diffracted light of interest propagates back along the propagation axis of the incoming beam. Those skilled in the art often use the term Littrow configuration more broadly to define an arrangement wherein some of the diffracted light beams of interest propagate close to the propagation axis of the incoming beam. The specification and claims will use the term Littrow configuration (or arrangement or condition) as it is more broadly defined. As an alterative to using this term, a compact spectrometer arrangement can also be defined as a configuration where a common lens is used to focus both the incoming and diffracted beams of interest.
In a basic Littrow configuration, the small separation between the incoming and diffracted beams of interest can result in some spatial overlap, making detection schemes more difficult to implement. This difficulty can be overcome by tilting or tipping the grating so that the incident wave vector of the incoming beam strikes the grooves of the grating at a non-normal angle. In this configuration, conical diffraction is created which results in the diffracted beam being separated from the propagation axis of the incoming beam. This separation is along an axis perpendicular to the plane defined by the incoming and diffracted beams and, for simplicity, will sometimes be referred to herein as a vertical separation of the beams. Even though the tipping of the grating results in the vertical separation of the diffracted beam from the incoming beam, those skilled in the art still generally refer to this arrangement as a Littrow configuration. The specification and claims will use the phrase “substantially Littrow” to cover all variants of the Littrow configuration (both in-plane and conical) and to distinguish the configuration from the prior art spectrometers which had large angles between the incoming beam and the diffracted beams being measured.
Littrow spectrometers have been used in the prior art. Most are of the classical in-plane diffraction design (see, U.S. Pat. No. 6,757,113). Littrow spectrometers have also been suggested that use conical diffraction to create vertical separation between the incoming and diffracted beams to be detected (see, U.S. Pat. No. 6,710,330). These designs still permit both the incoming and diffracted beams to be measured to share a common lens. As a result, the spectrometer can be compact with a substantially reduced size. This Littrow configuration also allows the spectrometer to be made highly stable to withstand mechanical vibration and temperature variation.
On problem with the prior art Littrow spectrometers which utilize conical diffraction is that the conical diffraction creates certain distortions and non-linearities in the focused beam. These problems are described in greater detail below with respect to FIG. 3. One aspect of the subject invention is to provide optical correction for such distortions.
It should also be noted that although a grating in a Littrow configuration has been reported in some OCT systems, these systems are not spectral domain OCT systems. Instead, the in-plane Littrow diffraction is employed to generate a reference beam with laterally displaced multiple optical path lengths and it did not use conical diffraction. See, for example: U.S. Pat. No. 6,847,454, and Zeylikovich, I. et al. (1998), “Nonmechanical grating-generated scanning coherence microscopy,” Optics Letters 23(23): 1797-1799), both of which differ from the present invention wherein a Littrow spectrometer is used in an SD-OCT system to disperse the interfered beam in the detection arm into its spectral components and to focus the dispersed spectral components onto a detector array.